As we all know by now, ducks have penises. Rather epic penises, in fact. Chickens, though, are penis-less. In fact, most birds don't have them. In an important update in duck sex news, Ed Yong follows the work of several scientists who are trying to better understand how genitals evolve and why they differ so much between species and genuses. Bonus new fact: A dissected goose penis looks surprisingly like a less-colorful Man-O-War jellyfish.